1. Field of the Invention
The present invention relates to thermosensitive recording materials and more particularly, to improved thermosensitive recording materials utilizing a color forming reaction between electron donating dye precursors which are substantially colorless or light-colored at normal temperature and electron accepting compounds.
2. Related Art
Thermosensitive recording materials of two component color forming system comprising a substantially colorless or light-colored dye precursor (hereinafter referred to as a color former) and an electron accepting compound (hereinafter referred to as a color developer) involve primary color formation and have many advantages in that post treatment such as development or the like is unnecessary, handling is easy, etc. Therefore, the system has become the main trend of thermosensitive recording materials.
Thermosensitive recording materials have been widely used for facsimiles, recording labels for measuring instruments, etc. In particular, a growth in the field of facsimiles is remarkable in recent years and with the growth, quality demand in recording materials for facsimiles is becoming severe.
That is, as the speed of facsimiles increases, recording in a short pulse width, namely, color formation in a sufficient density even with a small heat energy but no color formation by heat from a thermal head after printing, is required. In addition, adhesion of thermal melt products (background stains) on a thermal head should also be reduced as less as possible.
To comply with the above requirements contradictory to each other, it has been proposed to provide, for example, an intermediate layer comprising inorganic powders having an oil absorbing amount of more than 50 ml/100 g disclosed in Japanese Patent Application Laid-Open No. 54-23545; a layer comprising an expanded filler obtained by expanding an expandable plastic filler disclosed in Japanese Patent Application Laid-Open No. 59-225987; etc.
However, in the case of using thermosensitive recording materials for forming a color with a low heat energy, adherence of thermal melt to a thermal head tends to be increased. Where printing is made over long periods of time, printed matters become unclear due to head stains and printing fails sometimes in a serious case. Further where the quality associated with generation of stain is good, a decrease in density is large because of permeation of thermal melt when printed with a higher energy.